International Normalized Ratio
Does this test have other names?
INR, standardized prothrombin time
What is this test?
This blood test looks to see how well your blood clots.
The international normalized ratio (INR) is a standardized number that's figured out
in the lab. If you take blood thinners, also called anti-clotting medicines or anticoagulants,
it's especially important to check your INR. The INR is figured out using the results
of the prothrombin time (PT) test. This measures the time it takes for your blood
to clot. The INR is an international standard for the PT.
Why do I need this test?
You might need this test if you take drugs that change the way your blood clots, such
as warfarin for heart problems or heparin to prevent a blood clot. Anti-clotting medicines
are helpful if you are at risk for a stroke. But if you're in an accident and your
blood doesn't clot quickly, you may be at risk of dangerous bleeding. Your healthcare
provider uses the INR to know whether your anti-clotting drugs are on target or whether
you need a change in your dosage. Newer anti-clotting medicines do not need INR monitoring
and dose changes like warfarin, so this lab test may not be used for those medicines.
You may also need this test to help your healthcare provider diagnose and manage certain
health problems, such as liver disease and bleeding disorders.
What other tests might I have along with this test?
Your healthcare provider will order the PT test first, so that the results can be
used to calculate the INR. In a PT test, chemicals are added to your blood sample,
and the lab measures the time in seconds that it takes to clot.
Your provider may also order these tests if he or she is concerned about your liver
function or risk of bleeding:
What do my test results mean?
Many things may affect your lab test results. These include the method each lab uses
to do the test. Even if your test results are different from the normal value, you
may not have a problem. To learn what the results mean for you, talk with your healthcare
The INR is a ratio, so it's just a number, not a number tied to time or another value.
Your results will vary according to your age, the medicines you take, and any health
problems you have. In general, the higher the number, the longer it takes for your
blood to clot.
Your INR number should be between 2 and 3 if you are taking a blood thinner, but it
could be different, depending on your condition. For instance, your INR goal if you
are being treated for a mechanical heart valve will be different than your INR for
a clot in your leg. The INR goal is usually higher for mechanical heart valves, especially
the mitral valve. That goal is 2.5 to 3.5. Talk with your doctor about what your INR
goal is and what it means for you.
How is this test done?
The test requires a blood sample, which is drawn through a needle from a vein in your
Does this test pose any risks?
Taking a blood sample with a needle carries risks that include bleeding, infection,
bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight
stinging sensation or pain. Afterward, the site may be slightly sore.
What might affect my test results?
Certain foods and any medicines you take may affect your results, especially if you
take warfarin, heparin, or other anti-clotting drugs or are being treated for liver
disease with vitamin K. It is very important to follow your healthcare provider's
instructions for having this test.
How do I get ready for this test?
You may be asked to stop eating foods that are high in vitamin K before this test.
In addition, be sure your healthcare provider knows about all medicines, herbs, vitamins,
and supplements you are taking. This includes medicines that don't need a prescription
and any illicit drugs you may use.